Pretty much there are 3 important things to getting the best growth in your spawns...Good quality foods, a varied diet, and clean water.

Quality Foods and varied diet

1fish2fish pretty much covered this in another thread.... Here's the linkhttp://www.bettafish.com/showthread.php?t=51144 . It is really important to feed have a varied diet and quality foods. It is also best to feed the fry 2-3 small servings a day, instead of 1 big meal.

Clean Water

I feel this is the most important factor for good growth. There are speculations that Betta fry excrete a hormone that stunts the other fry's growth. If you change most of the water each day (About 50-75% of the water) you get rid of most of the hormones. For best growth,changing 50-75% of the water EVERY DAY after the second week is your best bet. I also found out with my most recent spawn,that if you slowly start adding water after you take the male out of the tank, there is less die off.(*Make sure the water is either aged or treated with conditioner and about the same temperature as the tank water) I started adding around 1.5 gallons of water every day, since I took the male out, and my fry have been growing at a nice rate.

It is also a good idea to siphon out uneaten food everyday. It is very important to keep those tank bottoms clean. I have read articles that say that if the bottoms become dirty,ithey could build up bacteria, which could lead to missing ventral fins.

All in all if you feed the right foods, and do daily water changes, you could have full grown fry at around the age of 3-4 months.

About the microworms.. and I mentioned this in my food thread but I'll mention it again. Eating the microworms is not what is thought to cause missing vents, it's letting the uneaten worms lie at the bottom of the tank. So having fry that spend their time at the bottom of the tank and not doing daily water changes may cause missing ventrals. Some breeders religiously feed MW without any problems which suggests it's a water quality issue and not a nutritonal issue.

Also, missing vents are a typical problem in fish that are too inbred. They may miss one vent or both or have two on one side, etc.

As far as algae, I don't think it will do any harm to the fry. It might even be beneficial because the algae (being a plant) will be able to absorb some of the waste products and keep the water cleaner.